A rock that lay in the centre of the Ring of Isengard, described as an island or peak rising out of the smooth floor of Isengard'sCircle. In the earliest days of Gondor, its people had shaped the rock and built the Tower of Orthanc above it, rising out of the rock itself. They left the Orthanc-rock impregnably smooth and hard (so hard in fact that it was speculated that they had placed an enchantment on the rock, as even the Ents could not break it).

Descriptions of the Orthanc-rock vary somewhat. In some cases the Tower of Orthanc is described as being built literally on the rock ('Saruman has a tower on it.'1), but elsewhere the Tower is formed directly from the rock itself ('A peak and isle of rock [Orthanc] was, black and gleaming hard...'2 In fact, Tolkien's conception of the Orthanc-rock changed over time, as we can see from his various illustrations of it. The earliest versions show a simple rock with a conventional tower built directly on it, but in later versions the Orthanc-rock has grown to immense proportions with the Tower of Orthanc appearing tiny on its summit. In later conceptions still, the rock and Tower have merged, so the Tower now rises directly from the plain, shaped from the rock rather than built on top of it, so '...it seemed a thing not made by the craft of Men, but riven from the bones of the earth...'.2

This sequence of illustrations is reproduced in Hammond and Scull, J.R.R. Tolkien: Artist and Illustrator.